The cardinality of a many-worlds multiverse isn't clear. It might be finite if all physical systems are in a superposition of a finite number of eigenstates in

The cardinality of a many-worlds multiverse isn't clear. It might be finite if all physical systems are in a superposition of a finite number of eigenstates in the collapse/split basis. You're also not creating anything out of nothing. The correct version of your complaint is "Let's create x > n things out of n things" (where "things" are quantum states), which happens all the time when you take the tensor product of two quantum systems, evolve the new overall system, and end up with a state that's not factorizable into a tensor product of two states.